Fragile X syndrome is a leading cause of mental retardation in humans. The molecular basis of fragile X syndrome is the expansion of a trinucleotide repeat within the FMR1 gene, leading to the absence of the encoded protein, FMRP. Although there has been a spectacular increase in the understanding of this disorder in recent years, it is now time for a well focused, multidisciplinary group effort to further elucidate the molecular basis of fragile X syndrome. Seven interrelated and collaborative project at Emory University School of Medicine are proposed. These projects dramatically expand the scope of contemporary fragile X syndrome research and specifically address issues crucial for future investigation and intervention. Clues to the mechanism(s) of repeat expansion will be sought and exploited. The consequence of repeat expansion, the transcriptional suppression of FMR1, will be investigated to understand the mechanism leading to the absence of FMRP. Understanding the biochemistry of FMRP will be highlighted with studies aimed at understanding the structural determinants of FMRP:RNA interaction, including the role of FMRP isoforms; the role of FMRP on protein translation and the consequence of its absence on neuronal protein synthesis; the localization of FMRP in mammalian brain, particularly within neuronal somatal-dendritic compartments, and the consequence of its absence on dendritic complexity and dendritic spine morphology. Model systems are proposed to further drive these studies to greater levels of understanding. A model system will be developed in yeast investigating yeast genes whose products are of similar structure to FMRP and the result of vertebrate FMRP expression within yeast will be investigated. A mammalian cellular system to assess FMRP function will be developed based upon the ability of excess FMRP to transform 3T3 cells in culture. Finally, a new generation of Fmr1 knockout mice will be developed in which FMRP expression can be temporally and quantitatively controlled, both pre- and postnatally, by drug exposure. Fundamental questions relating to future therapeutic strategies will be directly addressed. It is anticipated that this proposed investigation into fragile X syndrome by a highly interactive and multidisciplinary team of proven collaborative abilities will result in a synergistic increase in knowledge of this important disorder and a broader understanding of a common form of mental retardation.